Network types

Ethernet is a broadcast network type. Hence, Ethernet has the ability to do ARP broadcasts to find out what hosts are on the network. Other network types exist, such as point-to-point.

A broadcast network has other capabilities. For example, a host can send a packet to all other hosts within the LAN segment (known as a network broadcast), or the host can target a subset of all other hosts on the LAN (known as a network multicast).

A point-to-point network network, as the name suggests, consists of only two hosts, one at each end of the network. Broadcasting is not possible or required because there is only one other host within the network.

The z/OS host supports point-to-point interfaces in various contexts. In addition, a point-to-multipoint type network is also possible, particularly in a sysplex (which is discussed in the topic on TCP/IP in a sysplex). A point-to-multipoint network could be considered a hybrid: there are many hosts directly attached within the scope of a single network ID. However, there are no broadcast capabilities.

Local area network (LAN)

The term local area network is usually defined by its size: it is small and generally contained within a single room, a single building, or perhaps a small cluster of buildings. But perhaps a more accurate definition of the term LAN would be to refer to it as a physical segment within the scope of an ARP broadcast. For clarity, the term LAN segment will be used this way in the following information.

This not a cumbersome definition, however. There are tens of millions of computers attached to the network. How could an ARP broadcast possibly span all those computers every time it needs to send a packet to a particular IP address? The answer is that ARP broadcasts do not leave the physical LAN segments to which they are attached (you can read about the exceptions to this in the following note box).

Historically, a LAN segment is connected using an Ethernet hub. The hub is a layer 1 device only and thus it will repeat (transmit) any ARP packets to all hosts connected to the hub. Any network devices at the higher layers will not forward an ARP request.

Note: There are intelligent hubs (called network switches) that operate at layer 2. They may be configured as either ARP repeaters or they can cache ARP data relating to each LAN segment connected to the switch.

There are also devices called bridges (or repeaters) that operate at layer 1 and can seamlessly extend LAN segments.

Wide area network (WAN)

To summarize, at layer 3 an IP address is used to locate every host on the network. But hosts are located at layer 2 by a MAC address, not an IP address. Consequently, layer 3 uses ARP broadcasts to solicit a mapping of IP addresses to MAC addresses.

However, we have distinctly stated that the scope of an ARP address is within a LAN segment itself--unless, of course, a network switch (layer 2) or bridge (layer 1) is available to extend the scope of the segment.

For the sake of discussion, the term wide area network (WAN) will be used to denote a group of two or more local area networks connected at layer 3. A WAN would include the link (usually a high speed link) that is used for the interconnection of local area networks.

Other definitions of area networks exist that do not have a bearing on this information.